Most nitrogen is found in the atmosphere. The nitrogen cycle is the process by which atmospheric nitrogen is converted to ammonia or nitrates.
Nitrogen is essential to all living systems. To become a part of an organism, nitrogen must first be fixed or combined with oxygen or hydrogen.
Nitrogen is removed from the atmosphere by lightning and nitrogen-fixing bacteria. During electrical storms, large amounts of nitrogen are oxidized and united with water to produce an acid which is carried to the earth in rain-producing nitrates. Nitrates are taken up by plants and are converted to proteins.
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Then the nitrogen passes through the food chain from plants to herbivores to carnivores. When plants and animals eventually die, the nitrogen compounds are broken down giving ammonia (ammonification). Some of the ammonia is taken up by the plants; some is dissolved in water or held in the soil where bacteria convert it to nitrates (nitrification). Nitrates may be stored in hummus or leached from the soil and carried to lakes and streams. It may also be converted to free nitrogen (denitrification) and returned to the atmosphere.
The nitrogen cycle is one of the most difficult of the cycles to learn, simply because there are so many important forms of nitrogen, and because organisms are responsible for each of the introversions. Remember that nitrogen is critically important in forming the amino portions of the amino acids which in turn form the proteins of your body. Proteins make up skin and muscle, among other important structural portions of your body, and all enzymes are proteins. Since enzymes carry out almost all of the chemical reactions in your body, it’s easy to see how important nitrogen is. The chief reservoir of nitrogen is the atmosphere, which is about 78% nitrogen… Nitrogen gas in the atmosphere is composed of two nitrogen atoms bound to each other.
It is a pretty non-reactive gas; it takes a lot of energy to get nitrogen gas to break up and combine with other things, such as carbon or oxygen. Nitrogen gas can be taken from the atmosphere (fixed) in two basic ways. First, lightning provides enough energy to “burn” the nitrogen and fix it in the form of nitrate, which is nitrogen with three oxygen’s attached. This process is duplicated in fertilizer factories to produce nitrogen fertilizers. The other form of nitrogen fixation is by nitrogen-fixing bacteria, which use special enzymes instead of the extreme amount of energy found in lightning to fix nitrogen. These nitrogen-fixing bacteria come in three forms: some are free-living in the soil; some form symbiotic, mutualistic associations with the roots of bean plants and other legumes (rhizobial bacteria); and the third form of nitrogen-fixing bacteria are the photosynthetic cyanobacteria (blue-green algae) which are found most commonly in water.
All of these fix nitrogen, either in the form of nitrate or in the form of ammonia (nitrogen with 3 hydrogen’s attached). Most plants can take up nitrate and convert it to amino acids. Animals acquire all of their amino acids when they eat plants (or other animals). When plants or animals die (or release waste) the nitrogen is returned to the soil. The usual form of nitrogen returned to the soil in animal wastes or in the output of the decomposers is ammonia. Ammonia is rather toxic, but, fortunately, there are nitrite bacteria in the soil and in the water which take up ammonia and convert it to nitrite, which is nitrogen with two oxygens.
Nitrite is also somewhat toxic, but another type of bacteria, nitrate bacteria, takes nitrite and converts it to nitrate, which can be taken up by plants to continue the cycle. We now have a cycle set up in the soil (or water), but what returns nitrogen to the air? It turns out that there are denitrifying bacteria that take the nitrate and combine the nitrogen back into nitrogen gas.
The carbon cycle is relatively simple. From a biological perspective, the key events here are the complementary reactions of respiration and photosynthesis. Respiration takes carbohydrates and oxygen and combines them to produce carbon dioxide, water, and energy. Photosynthesis takes carbon dioxide and water and produces carbohydrates and oxygen. The outputs of respiration are the inputs of photosynthesis, and the outputs of photosynthesis are the inputs of respiration. The reactions are also complementary in the way they deal with energy. Photosynthesis takes energy from the sun and stores it in the carbon-carbon bonds of carbohydrates; respiration releases that energy. Both plants and animals carry on respiration, but only plants (and other producers) can carry on photosynthesis.
The chief reservoirs for carbon dioxide are in the oceans and in rock. Carbon dioxide dissolves readily in water. Once there, it may precipitate (fall out of solution) as a solid rock known as calcium carbonate (limestone). Corals and algae encourage this reaction and build up limestone reefs in the process. On land and in the water, plants take up carbon dioxide and convert it into carbohydrates through photosynthesis. This carbon in the plants now has 3 possible fates. It can be liberated to the atmosphere by the plant through respiration; it can be eaten by an animal, or it can be present in the plant when the plant dies. Animals obtain all their carbon in their food, and, thus, all carbon in biological systems ultimately comes from plants (autotrophs).
In the animal, carbon also has the same 3 possible fates. Carbon from plants or animals that are released to the atmosphere through respiration will either be taken up by a plant in photosynthesis or dissolved in the oceans. When an animal or a plant dies, 2 things can happen to the carbon in it. It can either be respired by decomposers (or released to the atmosphere), or it can be buried intact and ultimately form coal, oil, or natural gas (fossil fuels). The fossil fuels can be mined and burned in the future; releasing carbon dioxide into the atmosphere.
Otherwise, the carbon in limestone or other sediments can only be released into the atmosphere when they are subducted and brought to volcanoes, or when they are pushed to the surface and slowly weathered away. Humans have a great impact on the carbon cycle because when we burn fossil fuels we release excess carbon dioxide into the atmosphere. This means that more carbon dioxide goes into the oceans, and more is present in the atmosphere. The latter condition causes global warming because the carbon dioxide in the atmosphere allows more energy to reach the Earth from the sun than it allows escaping from the Earth into space.
The concentration of carbon in living matter (18%) is almost 100 times greater than its concentration in the earth (0.19%). So living things extract carbon from their nonliving environment. For life to continue, this carbon must be recycled. That is our topic.
Carbon exists in the nonliving environment as:
- carbon dioxide (CO2) in the atmosphere and dissolved in water (forming HCO3-)
- carbonate rocks (limestone and coral = CaCO3)
- deposits of coal, petroleum, and natural gas derived from once-living things
- dead organic matter, e.g., humus in the soil
Carbon enters the biotic world through the action of autotrophs:
- Primarily photoautotrophs, like plants and algae that use the energy of light to convert carbon dioxide to organic matter.
- And to a small extent, chemoautotrophs – bacteria and archaeans that do the same but use the energy derived from oxidation of molecules in their substrate.
Carbon returns to the atmosphere and water by
- respiration (as CO2)
- Decay (producing CO2 if oxygen is present, methane (CH4) if it is not.
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